Oscillations in Gas Channels II. Unraveling the Characteristics of the Low Frequency Loop in Air-Fed PEFC Impedance Spectra

  • Schneider I
  • Freunberger S
  • Kramer D
 et al. 
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Theeffect of oxygen depletion along the cathode flow channels onthe local impedance response of a polymer electrolyte fuel cell(PEFC) operated on air and pure hydrogen was investigated usinga novel experimental approach. The method combines the use ofsectioned electrodes and local ac measurements in PEFCs. The resultsshown here give experimental proof that oxygen concentration oscillations inthe cathode gas channels generated as a consequence of accurrent flow evoke the formation of a low frequency capacitiveloop in local H2/air-PEFC impedance spectra in downstream parts ofthe flow field. The loop shows positive or negative realparts at low frequencies depending on the air stoichiometry. Theloop is not observed in the local spectrum when theac current is applied locally and, consequently, the impact ofupstream processes on the local impedance response is excluded. Amechanistic description of the processes occurring during ac measurements inH2/air-PEFCs was developed and validated by experiments. The results showthat processes occurring along the flow channels of a PEFCcannot be excluded from the interpretation of PEFC impedance spectraeven at rather high air stoichiometries and are in factthe most important phenomena at low modulation frequencies (fmod

©2007 The Electrochemical Society

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  • I. a. Schneider

  • S. a. Freunberger

  • D. Kramer

  • A. Wokaun

  • G. G. Scherer

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